Expert: Dan Merrill - 10/12/2009

Question
QUESTION: I have an electric attic roof fan that when running appears to pull air out of the house.  The house has a gambrel roof.  There are no soffit vents with this house and the gable end vents are the old style with only slits in the aluminum siding.  
I also have three 12X12 inch roof can vents.
I am assuming I need to add some some additional ventilation but I am not sure what is the best way to go or how much I will need.
Any help is appreciated

ANSWER: If you are replacing your roof I would recommend products from DCI products or TrimLIne. They both offer drip edge venting. You cut an area out of the roof deck (plywood) that is about ¾” wide and install the vent over it. The vent allows intake air to be drawn in from the lowest accessible area of the roof. This is real handy when you have a roof without Soffit venting and without access to install soffit venting. This venting should be installed the full length of the eaves.

When you have an attic system without lower intake venting, there will always be problems. Air will always take the path of least resistance. So when you have a combination of Gable vents and a powered attic vent, the Gable vents will be the intake. This will reduce some of the heat in summer, but not a lot. The biggest issue is that this combination of vents will help very little in winter. The air can’t flow under the roof areas that need it the most.
Some old homes just can’t be ventilated enough to function well with our modern lifestyle changes. The volume of moisture released by an average family is many, many times what these homes were designed to tolerate. A hundred years the norm was one bath a week. And family members commonly used the same bath water. Homes had no insulation at all so the moisture could easily escape.

Today’s families release maybe 10 times to 20 times the moisture as compared to 100 years ago.
The best solution we have seen for many of these old homes is the newer building code allowance of “conditioned attic space”.   This idea is to spray closed-cell urethane foam on the underside of the roof deck (plywood) for insulation. This foam is a vapor and air barrier. Several inches of this material will prevent moist air from getting anywhere near cold surfaces so there is no problem with condensation.

This also keeps insulation away from problem areas like the remnants of old knob & tube wiring that can be a fire hazard if covered with insulation.
Retrofitting this insulation isn’t cheap but it is amazing how much it can improve an old homes comfort. The payback from reduced utilities costs is commonly 5-7 years in our experience.
Good luck,
Dan


---------- FOLLOW-UP ----------

QUESTION: Thanks you for your help.  I just want to clarify what “conditioned attic space” means.  Does this mean you are letting heat radiate out of the living space to temper your attic?  I have all of the second floor ducts running through the attic space.  We had to go this route because we had electric base board heat when we bought the house and we replaced it with geo-thermal.  In the winter, ice dams form.

Answer
Yes, generally speaking, your attic would be heated and cooled as the rest of the home. Doing this would also save you some of the energy you are presently losing due to your HVAC system being located in your attic.

Your ice damming issues are caused by the excess heat loss in your attic space. The heat rises, melts the snow and it flows down until it contacts a colder surface which would be above your soffit area. The foam will also eliminate this.

Make sure you get at least 3 quotes and only get quotes for closed-cell foam. Prices vary a lot. Open-cell foam will not do everything you need, it would be a waste of your money.
As an added bonus, Closed-cell urethane spray foam increases the strength of your structure usually 200-300%. There is just no better product to use when renovating older homes.

Generally speaking the spray closed-cell urethane foam will be one of the most expensive insulation products you can install. But it has many, many benefits. The product will last the life of the home, it does not age and settle as other products do.

Good luck,

Dan